Typical submicron integrated circuit manufacturing processes include, for example, the definition of isolation oxides, sometimes called field oxides 13 (FIG. 1) upon a substrate 11. The field oxides 13 define a comparatively flat region 15 of the substrate which lies between the field oxides 13. This flat region 15 is often termed the "moat". In the initial stages of integrated circuit fabrication, e.g. CMOS fabrication, the moat is covered with a sacrificial grown oxide 17.
After formation of the field oxide 13 and sacrificial oxide 17, one or more ion implantation steps is performed, for example, to define either a p-tub or an n-tub depending upon the species of implanted ions 19, or to adjust the threshold voltage of subsequently formed devices, e.g. transistors. Illustrative implantation species are arsenic, phosphorus, boron and boron difluoride.
The ion implantation step tends to change the stoichiometry and density of the upper portion 21 of the field oxide 13 causing this upper portion 21 of the field oxide 13 to etch more quickly when exposed to a wet etch solution, e.g. HF, than oxides which have not been influenced or damaged by ion implantation.
After dopant species 19 has been implanted into the substrate, a wet etch is performed to remove the sacrificial oxide 17 thereby exposing the ion implanted substrate 11 for subsequent processing, such as the formation of a gate oxide layer (not shown). At the same time that the sacrificial oxide is being removed by the wet etch, the etch solution also attacks the ion implant-altered upper surface 21 of the field oxide 13, thereby etching this surface downward to a location approximately indicated by reference line 23.
The exposure of the silicon substrate originally under the field oxide 13 is sometimes called "bird's beak pull back". This pull back is undesirable in that it reduces the vertical height of the field oxide 13 and often exposes a portion 25 (FIG. 2) of the substrate 11 which originally lay under the field oxide 13 and which has a different dopant concentration than the adjacent moat area 15. The height reduction of the field oxide 13 is particularly undesirable for submicron devices which use comparatively thin field oxides. Such thinning of the field oxide adversely affects the effectiveness of the field oxide in inhibiting parasitic transistor action and reduces the ability of the field oxide to block source-drain ion implantation. The above mentioned problems adversely affect device yield as well as device performance. The enhanced field oxide etch rate also enhances bird's beak pullback.
Those concerned with integrated circuit technology fabrication have sought more reliable methods of fabrication including methods to substantially reduce bird's beak pull back.